Your coolant should be a 50/50 mixture of water and antifreeze. Over time the additives in the coolant break down, and must be replaced. These additives help cool the engine, lubricate the waterpump, and prevent corrosion. The 'antifreeze' in your coolant is very corrosive, and so is the water you put in their, which is why it is so important to change your coolant at the recommended time, and to make sure your coolant is always full, if you get air in your radiator, not only does it not cool your engine efficiently, but it increases the rate of corrosion. I would get the radiator flushed and replace the cap, they wear out over time as well... Good luck.
